1. Field of the Invention
This invention relates generally to simulation of pyrotechnic shock for the purpose of testing electronic and other components, and more particularly, to a method and apparatus utilizing a resonant fixture capable of being tuned or conveniently reconfigured to simulate a range of different pyroshock conditions.
2. Description of the Invention
Satellite components as well as aerospace and weapon components are often subjected to pyroshock events during powered flight or deployment. As a result, system components must be qualified to this frequently severe environment. These shocks may be produced by explosive actuation devices such as detonators or linear explosives. Pyroshock-like environments can also be produced by high speed metal-to-metal impacts. The acceleration time history of a pyroshock resembles a decayed sinusoid with one or more dominant frequencies, and is characterized by high frequency, high amplitude, and a duration usually less than 20 msec. The net rigid body velocity change resulting from a pyroshock event is usually negligible. This environment is rarely damaging to structural elements, but can easily damage electronic components and assemblies.
The severity of a pyroshock environment is usually characterized using a shock response spectrum (SRS). The SRS used is normally the MAXIMAX spectrum which is the maximum absolute value acceleration response. An SRS is a plot of the maximum response of a single degree of freedom (SDOF) system as a function of the natural frequency of the SDOF. The magnitude of the SRS at a given frequency is the maximum absolute value response that would be produced on an SDOF system with the same natural frequency if it were subjected to the shock time history (base input). The SDOF damping ratio is a parameter which must be selected for the SRS calculation. This is normally chosen to be 5% for pyroshock data analysis.
The shock spectrum is viewed as a measure of damage potential. This is based on the assumption that the failure mechanism of a typical component can be modeled as a SDOF system. The application of the SRS as a design tool has historically resulted in robust component design for weapon and other components. When conducting pyroshock simulations, an SRS may be used as a means to quantifiably compare test environments with test requirements. In addition, efforts may be made to match time history peak G's, and total duration if these data are available.
Due to the high cost and complexity of most aerospace systems, component qualification using the actual pyroshock environment on complete assemblies is not reasonable. In addition, design margin cannot be determined with this approach. For these reasons, laboratory simulations of pyroshock environments are conducted on individual components and subassemblies. Traditional haversine pulse tests do not produce an adequate pyroshock simulation with regard to time history or SRS comparison. In general, the use of a haversine pulse test to simulate a pyroshock environment would result in a severe over-test at low frequencies, since the haversine test has considerably more velocity change than a pyroshock with comparable peak G's.
Presently, pyroshock environments are simulated in the aerospace industry by one of the following methods:
1. Electrodynamic Shaker.
This method can accurately produce a desired SRS within closely specified tolerances, but amplitude and frequency limitations of the equipment greatly restrict its applicability.
2. Live Ordnance with System Structure.
Since the actual system structure and live ordnance are used, this method has the potential to produce a shock virtually identical to the expected field environment. With this present technology, all the very high frequencies (&gt;10 KHz) associated with near-field pyroshock events are produced with this method. The cost of the test structure, however, is usually prohibitive, unless large numbers of identical tests are to be conducted. The use of live ordnance may have a wide repeatability tolerance, and does not easily allow the test levels to be increased so that an adequate design margin can be assured.
3. Live Ordnance with Mock Structure.
This method has most of the same features as 2, above, except that some cost savings are attributed to the use of a mass mock-up structure. These savings may be negated by the need for some trial-and-error testing to attain the desired component input, where geometric similarity was used in 2 to attain the same result.
4. Live Ordnance with Resonant Hate Fixture. This method further reduces test cost, and is a candidate for general purpose testing, due to the use of a generic resonant plate fixture. Since live ordnance is used, all the very high frequencies associated with near-field pyroshock events are produced with this method. However, a great amount of trial-and-error testing may be required to obtain the desired component input.
5. Mechanical Impact with Mock Structure.
Mechanical impacts do not produce the very high frequencies associated with the stress pulse in the immediate vicinity of a pyrotechnic device. However, most components in aerospace systems are isolated by enough intermediary structure such that the shock at the component location is not dominated by these very high frequencies. Instead, the shock at the component is dominated by the structural response to the pyrotechnic device, and has dominant frequencies which are typically less than 10 KHz. For these components, a mechanical impact (e.g. using a projectile or pendulum hammer) can produce a good simulation of the pyroshock environment. Test amplitudes can easily be increased or decreased by simply increasing or decreasing the impact speed. Frequency content can be controlled by the use of various pads affixed at the point of impact. Simulated pyroshock environments have been produced using mechanical impacts on system structures (or similar mass mock-ups). According to this method, the structure is impacted at the same point as the actual pyrotechnic device, and test conditions are experimentally adjusted so that the response at the component is appropriate. Due to the cost of the test structure, and the large amount of trial-and-error testing required, this method is impractical in most cases.
6. Mechanical Impact with Resonant Fixture.
In this method, a resonant fixture (typically a flat plate) is used instead of a mock structure. This significantly reduces cost, and allows for general purpose testing since the fixturing is not associated with a particular structural system. The mechanical impact excites the fixture into resonance which provides the desired input to a test component mounted on the fixture. Historically, test parameters such as plate geometry, component location, impact location, and impact speed, have been determined in a trial-and-error fashion. In general, this method produces a simulated environment which has its energy concentrated in a relatively narrow frequency bandwidth. This feature may not be desirable for some pyroshock environments. It should be noted here that a suitable resonant fixture for use in this method may also be a bar impacted either at the end or at some point along the length of the bar. The use of a bar-shaped resonant fixture is discussed in detail, below.
The methods just described are more fully explained in the following references: Daniel R. Raichel, "Current Methods of Simulating Pyrotechnic Shock", Pasadena, Calif.: Jet Propulsion Laboratory, California Institute of Technology, Jul. 29, 1991; Monty Bai, and Wesley Thatcher, "High G Pyrotechnic Shock Simulation Using Metal-to-Metal Impact", The Shock and Vibration Bulletin, Bulletin 49, Part 1, Washington D.C.: The Shock and Vibration Information Center, September, 1979; Neil T. Davie, "The Controlled Response of Resonating Fixtures Used to Simulate Pyroshock Environments", The Shock and Vibration Bulletin, Bulletin 56, Part 3, Washington D.C.: The Shock and Vibration Information Center, Naval Research Laboratory, August 1986; Neil T. Davie, "Pyrotechnic Shock Simulation Using the Controlled Response of a Resonating Bar Fixture", Proceedings of the Institute of Environmental Sciences 31st Annual Technical Meeting, 1985; "The Shock and Vibration Handbook", Second Edition, page 1-14, Edited by C. M. Harris and C. E. Crede, New York: McGraw-Hill Book Co., 1976; Henry N. Luhrs, "Pyroshock Testing--Past and Future", Proceedings of the Institute of Environmental Sciences 27th Annual Technical Meeting, 1981.
Much of the trial-and-error required with Method 6, above, has been eliminated by designing the resonant fixture such that its dominant lower mode or modes correspond to the dominant frequencies in the component test requirement. Using simple design principles, the fixture can be designed based only on the test requirement, and therefore, automatically has the desired frequency content. Minimal experimental adjustment is required to attain the proper amplitude and mechanical damping.
Existing pyroshock simulation technology according to Method 6, above, requires maintaining a large inventory of test fixtures in order to accommodate differing test requirements. In the alternative, resonant fixtures may need to be designed and built to custom specifications. Even given such test-specific preparations, trial-and-error is a significant factor in achieving desired testing conditions. All of these factors are costly and may result in difficulty in controlling test input.
In a recent U.S. Patent, the use of damping masses is described as a method to affect SRS in the context of a mechanical impact pyroshock simulator. U.S. Pat. No. 5,003,811, Shannon, et at., "Shock Testing Apparatus", discloses an apparatus wherein a longitudinal bar is impacted at one end and damping masses are clamped at preselected positions along the bar. According to the Shannon, et at., disclosure, an objective of the invention is to "tune" the resonant fixture in order to affect the SRS. Although Shannon, et al., seek to achieve a smoothing of the SRS for a given resonant fixture, their invention cannot use a single tunable apparatus to simulate different resonant frequencies.